U.S. Pat. Nos. 5,691,077 and 5,952,120 and U.S. Patent Publication No. 2007/0148538 disclose various methods for making dry process microporous battery separator membranes using uniaxial or machine direction (MD) stretching. The dry process (often referred to as the Celgard® process) may, in some instances, produce an elongated slit-shaped pore when using a uniaxial mode of stretching a nonporous, semi-crystalline, extruded polymer precursor. Battery separator membranes made using dry process uniaxial stretch methods may, in some instances, have a machine direction tensile strength higher than transverse directional tensile strength.
In order to increase the transverse directional tensile strength for various dry process porous separator membranes, U.S. Pat. No. 8,795,565 and U.S. Patent Publication Nos. 2011/0223486, 2014/0287322 and 2014/0302374 propose various methods including transverse direction (TD) stretching, performed simultaneously and/or sequentially with machine direction (MD) stretching (or uniaxial stretching).
U.S. Pat. No. 8,795,565 and U.S. Patent Publication Nos. 2014/0287322 and 2014/0302374 propose various transverse direction stretching methods which may include a simultaneous controlled machine direction relax step, a MD/TD stretch ratio of about 0.5 to about 4.0, and/or a MD tensile strength of up to about 1450 kg/cm2. The unique stretching processes may, in some instances, result in a change in the shape of the pores from an elongated slit-shape pore that may be found with some dry process uniaxial stretching processes, to a more round-shape pore (or a substantially round-shaped pore). Pore size and shape may be, in some instances, important separator membrane performance properties because they may influence the migration of electrolyte and ions between the electrodes of a lithium ion rechargeable battery during the charge and discharge cycles. U.S. Patent Publication No. 2011/0223486 proposes, in various embodiments, an adjustment in the amount of MD and TD stretching where the MD/TD stretch ratio is approximately equal to one, in order to produce a porous membrane with balanced MD and TD tensile strength. And in various instances, ER values of known biaxial stretched dry process battery separator membranes may be greater than 1 ohm-cm2.